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Ye Zhou, Xiangrun Huang, Wei Kong, Robert W. Knighton; Reflectance Decrease Prior to Thinning of Retinal Nerve Fiber Layer in Glaucomatous Retinas. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2443.
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Retinal nerve fiber layer (RNFL) consists of axons of retinal ganglion cells. Because glaucoma damages these axons, RNFL thickness, such as measured with OCT, is often used in clinic assessment of this damage. We have shown that glaucomatous damage alters these cylindrical structures before changes of RNFL thickness. In this study, we demonstrated that RNFL reflectance decreased prior to thinning of the RNFL and occurred first near the optic nerve head (ONH).
A rat model of glaucoma with laser photocoagulation of trabecular meshwork was used. Reflectance of RNFL in an isolated retina was measured at wavelengths of 400 - 830 nm. After reflectance measurements, the retina was stained with antibodies to label the axonal cytoskeleton. RNFL thickness (T) was measured by confocal fluorescence imaging. Reflectance relative to a diffuse white surface was calculated for bundle areas located at radii of 0.45, 0.8 and 1.0 mm from the ONH center. Because RNFL reflectance is very directional and wavelength dependent, bundles with peak reflectance and similar scattering geometry were selected and the mean reflectance (R) at different wavelength ranges was calculated. The reflectance ratio (P=R/T) was used for comparison. For glaucomatous eyes, only those bundles with no apparent structural damage were used.
Eighteen bundles of 10 control retinas and 21 bundles of 10 treated retinas were examined. Bundle thickness of both groups at each radius was not significantly different (p>0.2). However, in each wavelength range the reflectance ratio at 0.45-mm radius decreased significantly in the treated group, while the ratios were similar at the radii of 0.8 and 1.0 mm.
Elevation of intraocular pressure causes decrease of the RNFL reflectance that precedes change of RNFL thickness. The change may occur first near the ONH. The results suggest that decrease of RNFL reflectance near the ONH is an early sign of glaucomatous damage.
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